1. Field of the Invention
The present invention relates to a storage device, and in particular, to a storage device for storing information by changing resistance values of elements.
2. Description of the Related Art
Conventionally, storage elements are used to configure a storage device for storing information by changing their resistance values. One of such storage elements include variable resistive elements in which resistance values vary with the polarity of applied voltage. The variable resistive elements are switched to a high-resistance state when a voltage with a certain polarity applied, while being switched to a low-resistance state when a voltage with another polarity applied. For example, information can be stored through such correspondence wherein “1” and “0” correspond to the high- and low-resistance states, respectively.
In a storage device using such variable resistive elements, read operations may be performed using a voltage with polarity opposite to that of the applied voltage in write operation to the elements. In such cases, resistance values of variable resistive elements that vary with write voltage will vary with read voltage, so will the content of the written information. To reduce such variance in resistance values, some configurations are known for applying a read voltage as well as another voltage with inverse polarity and other configurations for changing polarity of a read voltage itself (see, Japanese Patent Laid-Open No. 2006-66052 and 2006-190376). However, such configurations require an additional circuit for controlling direction of applying read voltage. Thus, additional complexity is introduced in configuring a storage device including a power supply and other reading circuits, which may cause some problems.